The field of the invention relates to clothes dryers, and more particularly to dryers having controls other than straight timers for automatically shutting off the dryer.
It is well known that the optimum temperature and drying time for clothes varies greatly as a function of the fabric type and size of the load. For example, it is generally desirable to dry at a relatively high temperature so as to minimize the drying time, but some fabric types are damaged by hot temperatures. Also, different types of fabrics have different water storage capacities and different water removal rates. Accordingly, it has long been recognized that it is desirable to customize the temperature and drying time for a particular load of clothes.
Many prior art clothes dryers have one or more temperature sensors which are used to cycle the heater on and off so as to regulate the drying temperature. For example, a dryer would typically cycle on and off at a relatively high temperature (e.g. 150.degree. F.) for regular fabric or permanent press clothes and at a lower temperature (e.g. 130.degree. F.) for delicate fabric clothes. Most commonly, clothes dryers have a timer, and the operator sets the drying time (e.g. 45 minutes) after which the dryer shuts off. For this type of dryer, operator experience is required to set the time accurately. If too little drying time is selected, the clothes will not be dry and the operator will have the inconvenience of incrementally adding more drying time until the desired dryness is reached. If too much drying time is selected, then heat will be wasted and energy efficiency will be adversely impacted. In an attempt to improve and simplify the selection of drying parameters such as temperature and time, various prior art approaches have been tried.
One prior art automatic dryer approach uses a timer that only runs when the heater (i.e. electric, gas, steam, etc.) is turned off by the temperature regulating device. More specifically, different temperature thermostats corresponding to respective fabric types are located in the air exhaust duct, and the heater is cycled on and off at the temperature of the selected thermostat. The timer only runs when the heater is off, so the operator is actually selecting the "off time" to be expired before the dryer automatically shuts off. Initially, when the clothes are wet, much of the heat is consumed in evaporating large amounts of moisture. Accordingly, it takes a relatively long time for the exhaust air to reach the thermostat temperature in the first heating cycle, and during this time, the automatic timer is not running. Then, when the heater is turned off because the thermostat temperature has been reached, the timer starts to run and continues to run until the heater cycles on again, at which time it stops. This cycling process repeats until the timer setting, as selected by the operator, expires. Although this approach is easier to operate than a straight timer approach, there is still a large range of settings available to the operator, and some trial and error is generally required before the operator attains the experience to obtain optimum results.
Another prior art approach counts the number of on/off heater cycles, and after a predetermined number is reached, the dryer shuts off. More specifically, in one prior art commercial dryer, the operator selects between three settings that correspond to HIGH, PERM PRESS, and LOW temperature settings. The dryer includes a microprocessor and a look-up table matrix of settings such that, as preprogrammed by the owner of the dryer, each operator selected setting corresponds to a particular temperature and a particular number of heater cycles. For example, for the HIGH temperature selection, the owner may select between 190.degree. F., 185.degree. F., and 180.degree. F. as the cycling temperature. Also, for example, the owner may preprogram the auto dry level such that the microprocessor would count from 5 up to 14 on/off cycles before terminating the drying operation. As an illustration, if the owner had preprogrammed 185.degree. and 9 cycles for the HIGH setting and the operator selected the HIGH setting, the dryer would cycle on and off 9 times at an upper temperature of 185.degree. F., and then the dryer would be shut off. As with the previously described approach, this approach also has a drawback in that it relies on experience to customize the dryer setting for the particular clothes load. If the owner/operator has no experience and therefore guesses at selections, or relies on a default selections, the result could likely be over or under dried clothes. Once the owner/operator experiences this problem, compensation can be made for the next clothes load that is similar in nature. Eventually, the owner/operator can develop a proficiency with the control so as to compensate for various clothes loads so as to achieve the desired results. However, there is still a need to have a control that operates satisfactorily without individual experience.
The prior art also includes automatic control systems that measure the moisture content of the clothes. One type has a sensor in the exhaust duct that senses the moisture. The other measures the electrical resistance of the clothes in the dryer; as the clothes become dryer, their electrical resistance drops. These moisture sensors, however, are relatively expensive, and are subject to failure and miscalibration.